From DE 19740137 A1, a method and a circuit arrangement for generating AC ringing voltage for electronic subscriber circuits is known. In this arrangement, the operating voltage of a power amplifier is controlled in such a manner that the operating voltage follows the variation with time of an amplified AC ringing voltage in particular time intervals.
DE 19858963 A1 describes an audio signal amplifier with a controllable voltage supply for an amplifier stage and with an analyzing unit for analyzing the audio signal to be amplified. The audio signal amplifier also contains a control unit for controlling the voltage supply in dependence on the analyzed audio signal, and a delay unit 3 which delays the incoming audio signal relative to the audio signal supplied to the analyzing unit.
In conventional communication systems, direct voltage and low-frequency signals and high-frequency signals are frequently transmitted via one signal line or one transmission channel, respectively. The direct voltages are, for example, supply voltages for subscriber terminals. The low-frequency signals are, for example, analog voice signals or ringing tone signals for telephones. The high frequency signals to be transmitted by the communication systems are in most cases data signals such as, for example, xDSL data signals.
FIG. 1 diagrammatically shows an example of a communication system in which low-frequency and high-frequency signals are transmitted via the same line. From a voice and data network, low-frequency voice signals and high-frequency data signals are supplied to a switching center which has a switching device S, for example a multiplexer circuit. The switching device S is connected to a driver circuit T for a subscriber. Each subscriber connected to the switching center has his own driver circuit T. From the driver circuit T, the voice and data signals pass, for example, to a so-called splitter which consists of two filters, namely a high-pass filter HP and a low-pass filter TP. The low-pass filter TP is connected to a telephone which receives the low-frequency voice signals. The high-pass filter HP is connected to a data modem which demodulates and modulates the high-frequency data signals.
As a rule, the direct voltages which are transmitted from the driver circuit T to the circuit and are used, for example, to supply the telephone with voltage, and the low-frequency ringing tone have high signal amplitudes of more than 100 volts. This necessitates a very high supply voltage for the driver circuit T. The high-frequency signal currents which are used, for example, for transmitting data, have a much lower signal amplitude. Due to the lower-valued load impedance, the high-frequency signal currents produce large supply currents which are taken from the supply voltage of the driver circuit T, however.
FIG. 2 shows a driver circuit T of the prior art. The driver circuit T contains an operational amplifier OP which is supplied with voltage by an associated supply voltage source VB via two supply voltage lines. The input E of the driver circuit T is connected to a low-frequency signal source S1 and a high-frequency signal source S2, the signal currents of which are superimposed. The superposition of the various signal currents or signal components is indicated by a summing element in FIG. 2. The low-frequency voltage source S1 supplies the direct voltages for supplying the terminals and low-frequency signals, particularly low-frequency voice signals. The high-frequency signal source S2 delivers high-frequency data signals with a relatively low signal amplitude. At the output A of the driver circuit T, the entire signal spectrum is available at a low impedance for driving the terminals connected to the output.
As a rule, the supply voltage VB for the driver circuit T is supplied by a connected power supply.
The disadvantage of the driver stage T of the prior art as shown in FIG. 2 consists in that the supply voltage VB must be selected to be of such an amplitude that the maximum signal amplitudes of the two signal sources S1 and S2 are not limited in the operational amplifier OP.
The required supply voltage VB for the operational amplifier OP within the driver stage T can be calculated from the following relation:VB=Ŝ1+2=VDropwhere                Ŝ1 . . . crest value of the low-frequency signal currents,        2 . . . crest value of the high-frequency signal currents,        
VDrop . . . design-related voltage drop in the driver circuit.
The power dissipation PV of the driver circuit T, which is due to the load current is obtained from:PV=PG−PL=VB·F2·i1,2−PLwhere                PV load-current-related power dissipation in the driver circuit T,        PG load-current-related total power consumption of the driver circuit T,        PL power delivered to the load,        VB supply voltage of the driver circuit T,        i1,2 rms value of the total signal current,        F2 the form factor, i.e. the signal-shape-dependent ratio between rectified value and rms value of a signal.        
The power dissipation of the driver circuit T increases with increasing supply voltage VB of the driver circuit as can be seen from the equation. Since the supply voltage VB of the driver circuit T of the prior art is greater than the sum of the two crest values 1, Ŝ2 of the low-frequency and high-frequency signal currents, the necessary supply voltage VB of the driver circuit T, and thus the power dissipation PV of the driver circuit T are very high. In the conventional driver circuit T shown in FIG. 2, the power dissipation PV produced is of such a magnitude that it has hitherto not been possible to implement integrated driver circuits for full-rate ADSL in connection with an analog voice signal transmission without an elaborate housing with very large heat sinks.